An example of a conventional light-emitting device is described in U.S. Pat. No. 9,525,148 (Kazlas et al., issued Dec. 20, 2016). FIG. 1 is a drawing depicting an exemplary representation of such a conventional light-emitting device. A conventional light-emitting device includes an anode 104 and cathode 100, and a light-emitting or emissive layer 102 containing a material that emits light 107. Within the light-emitting layer 102, light is produced upon electron and hole recombination to generate the light 107. The light-emitting layer 102 may be an inorganic or organic semiconductor layer, or a layer of quantum dots (QDs). At least one hole transport layer 103 is located between the anode 104 and the emissive layer 102, which provides transport of holes from the anode and injection of holes into the emissive layer. Similarly, at least one electron transport layer 101 is located between the cathode 100 and emissive layer 102, which provides transport of electrons from the cathode and injection of electrons into the emissive layer. As is referred to in the art, a “top emitting light-emitting device” is a device in which the light emission is from the side opposite from a glass layer substrate upon which the other layers are deposited.
In such conventional structures, the layer (or layers) 101 between the cathode 100 and emissive layer 102 is termed the electron transport layer (ETL), and the layer (or layers) 103 between the anode 104 and the emissive layer 102 is termed the hole transport layer (HTL). The ETL and HTL are collectively referred to more generally as charge transport layers (CTL). The purpose of these CTLs is to provide an ohmic contact to the respective electrode (anode or cathode), and to provide energetic alignment for injecting carriers (electrons or holes) into the emissive layer. In conventional structures, the ETL is often comprised of a matrix of nanoparticles 108, which provides electron transport through hopping 109 between adjacent nanoparticles and into the emission layer 102. Similarly, the HTL is often comprised of a matrix of nanoparticles 110 (which typically are different from the ETL nanoparticles 109), which provides electron transport through hopping 111 between adjacent nanoparticles and into the emissive layer 102. As referenced above, the electrons and holes recombine within the emissive layer 102 to generate the light 107. In a conventional system only the ETL are nanoparticles, as TFB and PEDOT:PSS layers typically are not nanoparticle layers but continuous layers. The emissive layer is a QD layer, which is shown in FIG. 1 as the spheres. Nanoparticle materials also may be used for the HTL layer.
Because the electrodes (anode and cathode) are at least partially reflecting, an optical cavity is formed between the electrodes. Such cavities are well known in the art of semiconductor laser fabrication, as described for example in U.S. Pat. No. 7,324,574 (Kim, issued Jan. 29, 2008), although their use with organic LEDs is more recent. There are a number of descriptions for organic light-emitting diode (OLED) and QLED applications that describe cavities in the LED structure and the effect on light emission. For example, US 2006/0158098 (Raychaudhuri et al., published Jul. 26, 2006) describes a top emitting structure, and U.S. Pat. No. 9,583,727 (Cho et al., issued Feb. 28, 2017) and U.S. Pat. No. 8,471,268 (Moon et al., issued Jun. 25, 2013) describe an OLED and QLED structure, with light emitting regions between reflective areas, one of which is partially transmitting.
Examples of methods for improving the luminance of such cavities are described in the following. US 2015/0084012 (Kim et al., published Mar. 26, 2015) describes the use of dispersive layers in an OLED structure, U.S. Pat. No. 8,894,243 (Cho et al., issued Nov. 25, 2014) describes the use of microstructure scattering for improving efficiency, and WO2017/205174 (Freier et al., published Nov. 30, 2017) and U.S. Pat. No. 8,581,230 (Kim et al., issued Nov. 12, 2013) describe enhancement of the emission by use of surface plasmon nanoparticles or nanostructures in the transport layers. US 2014/0014896 (Chung et al., published Nov. 16, 2014) describes a QLED structure with a thick ETL layer, which is related to charge injection but does not specify generally an ideal thickness for overall enhanced performance and optical efficiency. US 2015/0340410 (Hack et al., published Jan. 26, 2015) and US 2017/0207281 (Hack et al., published Jul. 20, 2017) describe OLED color pixels purportedly with different optical path lengths, although there is no description about how this exactly would be achieved.
For many applications, typical QD emissive layers include cadmium selenide (CdSe) (or similar materials that include cadmium), as the properties of this material are such that the device design using such an emissive layer is similar to an OLED based structure. CdSe, however, contains cadmium, which can be extremely harmful to health and the environment, and therefore materials such as indium phosphide (InP) have been developed for use as an alternative material for the emissive layer which do not contain cadmium. Although non-toxic, these InP based materials have significantly higher absorption and refractive index as compared to CdSe, and this changes the nature and performance of the optical cavity. In particular, there is a substantially higher degree of reflection at the boundaries of the emissive layer and the CTLs. Accordingly, conventional configurations of top emitting QLEDs have not yielded comparable performance and optical efficiency when the emissive material is a material of high refractive index, such as a InP based material or comparable material.